1. Field of the Invention
The present invention relates to an electrophotographic photoconductor using an organic photoconductive material, and an image forming apparatus.
2. Description of Related Art
In recent years, an organic photoconductive material has been extensively researched and developed and has been applied to an electrostatic recording element, a sensor material, an organic electro luminescent (Electro Luminescent: EL) element or the like in addition to being used for an electrophotographic photoconductor (hereinafter, simply referred to as photoconductor in some cases) in a field of copying machines.
Conventionally, in addition to the field of the copying machines, the electrophotographic photoconductor using the organic photoconductive material has been used in fields of printing materials, slide films, microfilms and the like, in which a photographic technique has been used. The electrophotographic photoconductor has been also applied to a high-speed printer in which a laser, a light emitting diode (Light Emitting Diode: LED), a cathode ray tube (Cathode Ray Tube: CRT) or the like is used as a light source.
Therefore, requirements for the organic photoconductive material and the electrophotographic photoconductor using the organic photoconductive material are getting-higher and more extensive.
Conventionally, an inorganic photoconductor has been used as the electrophotographic photoconductor, comprising a photosensitive layer mainly containing an inorganic photoconductive material such as selenium, zinc oxide, cadmium or the like.
However, the inorganic photoconductor has problems such as difficulty in forming the photosensitive layer, low plasticity, high production cost and others although the inorganic photoconductor has essential properties to a certain extent as the photoconductor.
Also, generally, the inorganic photoconductive material is highly toxic and has great restrictions in production and handling.
On the other hand, an organic photoconductor using the organic photoconductive material has advantageous points such as lightweight, high translucency, easiness of designing the photoconductor having fine sensitivity to wide-ranged wavelengths by an adequate sensitization method and others in addition to easiness in film formability of the photosensitive layer and excellent flexibility. Therefore, the organic photoconductor tends to be gradually developed as a main force of the electrophotographic photoconductor.
Although the organic photoconductor in early years has had disadvantageous points such as sensitivity and durability, these disadvantageous points have been remarkably improved by development of a function-separation type electrophotographic photoconductor in which a charge generating function and a charge transporting function are allotted to respectively different substances.
The function-separation type photoconductor has advantageous points that the option of selecting materials for a charge generating substance allotted for the charge generating function and a charge transporting substance allotted for the charge transporting function is wide and that the production of the electrophotographic photoconductor having desired characteristics is relatively easy.
As the charge generating substance to be used for such a function-separation type photoconductor have been investigated many kinds of substances such as phthalocyanine pigments, squarylium coloring materials, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine coloring materials, squaric acid dyes and pyrylium type coloring materials, and various kinds of materials with high lightfastness and high charge generating capability have been proposed.
On the other hand, as the charge transporting substance have been known pyrazoline compounds (e.g. reference to Japanese Patent No. Sho 52-4188), hydrazone compounds (e.g. reference to Japanese Patent Application Laid-Open No. Sho 54-150128, Japanese Patent No. Sho 55-42380 and Japanese Patent Application Laid-Open No. Sho 55-52063), triphenylamine compounds (e.g. reference to Japanese Patent No. Sho 58-32372 and Japanese Patent Application Laid-Open No. Sho 54-151955), stilbene compounds (e.g. reference to Japanese Patent Application Laid-Open No. Sho 58-198043 and Japanese Patent Application Laid-Open No. Hei 2-190862) and the like.
Requirements for a charge transporting substance include:
(1) to be stable to light and heat;
(2) to be stable to active substances such as ozone, nitrogen oxide (NOx), and nitric acid generated by corona discharge at the time of charging a photoconductor surface;
(3) to have high charge transporting capability;
(4) to have high compatibility with an organic solvent and a binding agent; and
(5) to be produced easily at a low cost.
However, the charge transporting substances satisfy some of these requirements but cannot satisfy all of them at high level.
Among those five requirements, “to have the high charge transporting capability” of the item (3) is particularly required. This is because the charge transporting substance having the high charge transporting capability is required to obtain sufficient photo-response in the case of a charge transporting layer formed by dispersing the charge transporting substance with a binder resin being a surface layer of the photoconductor.
When the photoconductor is used on board of a copying machine, a laser beam printer or the like, a part of the surface layer of the photoconductor is unavoidably scraped by a contacting member such as a cleaning blade, a charging roller and the like. Therefore, for high durability of the copying machine and the laser beam printer, the strong surface layer of the photoconductor against the contacting member, that is, the surface layer which is hard to be abraded by scraping with the contacting member and which has high printing durability is required.
If the percentage of the binder resin content in the charge transporting layer which is the surface layer of the photoconductor is increased to improve reinforcement and durability of the surface layer, the photo-response of the charge transporting layer is decreased. This is because a ratio of the charge transporting substance in the charge transporting layer is lowered. That is, the charge transporting substance in the charge transporting layer is diluted as the percentage of the binder resin content is increased, and the photo-response of the charge transporting layer is deteriorated as a result of a decrease in the charge transporting capability of the charge transporting layer.
When the photo-response of the charge transporting layer is poor, residual potential rises and the photoconductor is used repeatedly in the state that surface potential is not sufficiently decayed. Therefore, a surface charge to be removed is not sufficiently eliminated by light exposure to result in undesirable consequence such as early deterioration of the quality of images.
Therefore, in order to obtain the sufficient photo-response, the charge transporting substance is required to have the high charge transporting capability.
Recently, the photoconductor has been required to have high sensitivity as a photoconductor characteristic corresponding to the demands for high speed as miniaturization and high speed of electrophotographic apparatuses such as digital copying machines and printers have been advanced. Also, it is required for the photoconductor to maintain sensitivity in a low-temperature environment and to ensure high reliability by controlling characteristic changes in various conditions.
Accordingly, the charge transporting substance is increasingly required to have high charge transporting capability. Also, in the high-speed process, since the time from exposure to development is short, it is required for the photoconductor to be excellent in the photo-response. However, as described above, since the photo-response depends on the charge transporting capability of the charge transporting substance, the charge transporting substance is required to have even higher charge transporting capability in terms of such a purpose.
Conventionally, as a purpose in developing the charge transporting substance satisfying the requirements has been molecularly designed in various forms and proposed compounds containing both the hydrazone structure and the styryl structure as compounds having more excellent capability in order to greatly expand a conjugated system in a basic structure (e.g. reference to Japanese Patent Application Laid-Open No. Hei 5-66587). However, if these compounds are used in a low-temperature environment, the sensitivity decreases, improvement is necessary in order for sufficient charge transporting capability, and the capability of the photoconductor is insufficient.